This patent specification is directed to a method for characterizing a structure on a mask.
This patent specification is also directed to a device for carrying out said method.
In the fabrication of semiconductor components, numerous measurement methods are used to monitor the results of the individual steps in the lithographic process. One fundamental task in this context is to measure the outer dimensions or shapes of the structures which are to be produced.
To check the quality of a process step, it is advantageous to examine the smallest imageable structures. These are referred to as the critical dimension, abbreviated CD. The critical dimension of a mask usually refers to the line width of a structure composed of alternating lines and spaces. In the lithographic process, this structure is first formed on a mask (photomask, reticle), for example in chromium on quartz glass. There are also other known masks, for example phase shift masks (PSM) or reflective masks, that are used particularly with short-wavelength illuminating radiation in the EUV range. By exposure in a scanner, this structure is imaged onto a wafer coated with resist. The desired structure is produced on the wafer by subsequent developing and etching.
The characterization of a structure, such as, for example, measurement of the CD or line width of a structure, can be performed both on the mask and on the wafer. On-wafer measurement does yield very meaningful data, since the final product is measured; it is very laborious, however, since the entire wafer exposure process has to be performed for the test.
If the characterization of a mask structure is performed on the mask itself, errors caused by the behavior of the mask during imaging and by other method steps are incorporated into the measurement. Errors on the mask are usually intensified by the scanner during imaging. Another problem is that the mask structures are known and are optimized by resolution enhancement technology (RET), and thus do not wholly match the structures that are to be imaged. This makes it difficult to measure the CD on the mask.
Both on mask and on wafer, CD measurement is performed by, for example, scanning electron microscopy (CD-SEM, critical dimension scanning electron microscopy).
Another option for characterizing masks is to analyze the aerial images of masks with a mask inspection microscope. In this method, the aerial image shows most of the features that are also projected onto the wafer.
Another option for characterizing masks and wafers is afforded by non-imaging optical methods. Various interrelated measurement methods are used for this purpose, such as, for example, conventional scatterometry, ellipsometry, diffractometry or reflectometry, known collectively under the generic term scatterometry. Masks can also be characterized by measuring mask transmission. The CD of a structure can also be determined by this method, as disclosed, for example, in EP 2171539 A1.